1. Field of the Invention
The present invention relates to methods for producing, from crude naphthalenedicarboxylic acid containing 6-formyl-2-naphthoic acid, high-purity 2,6-naphthalenedicarboxylic acid which is advantageously used as a material in engineering plastics, such as polyethylene naphthalate and polybutylene naphthalate. In particular, the present invention relates to a method for purifying crude 2,6-naphthalenedicarboxylic acid to obtain high-purity 2,6-naphthalenedicarboxylic acid. The method prevents corrosion of equipment and allows aftertreatment to be simple.
2. Description of the Related Art
Naphthalenedicarboxylic acid, and more particularly, 2,6-naphthalenedicarboxylic acid (2,6-NDA) is used as an important material in engineering plastics such as polyethylene naphthalate (PEN) and polybutylene naphthalate (PBN).
As shown in the following formula, 2,6-NDA is produced by oxidizing 2,6-dimethylnaphthalene (2,6-DMN) in a gas phase or a liquid phase.
Formula 
In a process using this oxidization reaction, however, the methyl group is not completely oxidized, and consequently 6-formyl-2-naphthoic acid (FNA) is produced as a by-product. When polyester, such as polyethylene naphthalate or polybutylene naphthalate, is produced from 2,6-NDA by polycondensation, FNA degrades the degree of polymerization and colors the polyester. Therefore, 2,6-NDA used for polyester must be of high purity, and specifically, it must not contain the impurity, that is, FNA.
Since 2,6-NDA and FNA have similar physical properties, such as boiling point and melting point, they cannot be separated by distillation or crystallization. Accordingly, various separation methods have been suggested. For example, low-purity 2,6-NDA produced by oxidization in a liquid phase is esterified with methanol (Japanese Unexamined Patent Application Publication No. 50-95253), and is subsequently distilled (Japanese Unexamined Patent Application Publication No. 50-29291) or recrystallized (Japanese Unexamined Patent Application Publication No. 50-111056) to prepare dimethyl 2,6-naphthalenedicarboxylate, which is a methyl ester. Then, the methyl ester is purified. In another method, crude 2,6-NDA is dissolved in an alkaline aqueous solution and is then decolorized (Japanese Unexamined Patent Application Publication Nos. 52-20993 and 52-20994, Japanese Examined Patent Application Publication Nos. 49-133359, 50-105639, 62-212341, and 62-212342), oxidized (Japanese Unexamined Patent Application Publication Nos. 48-68554, 48-68555, and 62-250849), or hydrogenated (Japanese Examined Patent Application Publication No. 57-36901).
In one of the above-described purification methods in which the methyl ester is purified, the carboxylic acid must be esterified with methanol in advance. This makes the purification process complex, and consequently the manufacturing cost increases. Also, in the other methods in which an alkali or an acid is used, purification equipment is readily corroded and separation of the acid or neutralization is required after the purification. Accordingly, these methods make aftertreatment complex and are, therefore, not cost effective.
Japanese Unexamined Patent Application Publication No. 9-151162 discloses that crude 2,6-NDA is reduced with hydrogen in the presence of a hydrogenation catalyst and is subsequently washed with a lower aliphatic carboxylic acid. In this method, the impurity FNA is reduced to 2-naphthoic acid (2-NA) with hydrogen, and then the naphthoic acid and methylnaphthoic acid (MNA) produced due to incomplete reduction are dissolved in a lower aliphatic carboxylic acid and then removed.
Unfortunately, this method needs a large amount of acid in the cleaning process, consequently causing the acid to corrode the equipment.
Accordingly, an object of the present invention is to provide a method for purifying crude 2,6-naphthalenedicarboxylic acid to produce high-purity 2,6-naphthalenedicarboxylic acid without damaging the equipment or increasing the cost, and making the purification process less complex.
The present invention is directed a method for purifying crude 2,6-naphthalenedicarboxylic acid including: a reduction step of reducing crude 2,6-naphthalenedicarboxylic acid containing 6-formyl-2-naphthoic acid with hydrogen to prepare a reduction product; and a cleaning step of cleaning the reduction product with alcohol.
Preferably, the reduction step includes a sub step of bringing the crude 2,6-naphthalenedicarboxylic acid and the hydrogen into contact with a hydrogenation catalyst in a liquid phase. Preferably, the liquid phase medium is water. The method may further include a dissolving step of dissolving the crude 2,6-naphthalenedicarboxylic acid and the hydrogen in the water. The dissolving step is performed before the reduction step. Preferably, the hydrogen concentration in the water is in the range of 10 to 100 ppm.
Preferably, the mass of the alcohol is in the range of 2 to 20 times the mass of the reduction product. Preferably, the alcohol is ethanol.